Fake news reports soar on social media, where links are given the same weighting regardless of source, and particularly on Facebook, where there is a potential audience of 1.89bn.
Photograph: Niall Carson/PA

Until recently, there was news and “not news” – as denoted by comments of “that’s not news” below the line on more light-hearted stories or features. Now there is “fake news”, said to be behind the election of Donald Trump as US president and a recent incident involving a gunman at a Washington pizzeria.

The term has become widely used – too widely. But it’s understandable there’s confusion when some fake news is only a bit fake, or fake for an arguably legitimate reason (such as satire).

Can we still make a useful definition of fake news? And should we even be worried about it at all?

‘Pizzagate’ gunman for days tried to recruit friends to join raid, affidavit says

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Definitely real fake news

First the most famous example of an indisputably (or so you would think) fake news story that has had real-world consequences.

Subscribers to “Pizzagate” point to apparent “code” within the hacked emails of John Podesta, the Clinton campaign chair, and the fact the restaurant has the same initials as “child pornography”. (Googling Pizzagate is not advised. The background to this extraordinary story has been explored in detail on the Reply All podcast.)

From 4chan and Reddit, the online message board where the story took root, the story gained sufficient momentum for a poll of 1,224 registered voters in late September to find that 14% of Trump supporters believed it to be true.

A significant 32% of respondents were “not sure” – just like the 25-year-old gunman who, though not convinced of the theory, felt there were sufficient grounds to pay a visit to the restaurant to “self-investigate”.

Yes, it stretches the boundaries of belief – but a lot of fake news does. And yes, people nonetheless think it’s true.

How big a problem is this kind of fake news, really?

The poll lends weight to the suggestion that the US election result was influenced by a widespread belief in fake news among Trump supporters. That same poll found that 73% of Trump voters thought the billionaire financier George Soros paid protesters to disrupt the Republican candidate’s rallies – a fake news report later repeated by the president-elect himself.

Another fake news report, that Democratic senators wanted to impose sharia in Florida, was repeated by Michael Flynn, Trump’s nominee for national security adviser. (Flynn also tweeted a “MUST READ” link about Democrats’ “sex crimes w children” – then deleted it.) A false report that Trump supporters chanting “we hate Muslims, we hate blacks, we want our great country back” at a rally was reported as true on election night.

These stories – compelling to click on, and with a “truthiness” quality to them – soar on the social web, where links are given the same weighting regardless of source, and particularly on Facebook, where there is a potential audience of 1.8bn.

Analysis by BuzzFeed found that fake news stories drew more shares and engagement during the final three months of the campaign than reports from (for example) the New York Times, the Washington Post and CNN.

Both Mark Zuckerberg and Sheryl Sandberg, the chief operating officer of Facebook, pooh-poohed suggestion that fake news travelling on the network could have swung the election. But, on Thursday, Facebook announced that it would begin flagging fake news stories with the help of users and outside fact-checkers.

Surely it’s easy to tell fake news from real news

Actually, no.

A recent study carried out by Stanford’s Graduate School of Education assessed more than 7,800 responses from middle school, high school and college students in 12 US states on their ability to assess information sources.

Researchers were “shocked” by students’ “stunning and dismaying consistency” to evaluate information at even as basic a level as distinguishing advertisements from articles.

Hosted on websites that often followed design conventions of online news media, with anodyne titles such as “Civic Tribune” and “Life Event Web” to give the semblance of legitimacy, the stories are geared to travel on social media.

Is The Onion fake news?

Kind of. Strictly speaking, fake news is completely made up and designed to deceive readers to maximise traffic and profit.

But the definition is often expanded to include websites that circulate distorted, decontextualised or dubious information through – for example – clickbaiting headlines that don’t reflect the facts of the story, or undeclared bias.

With nearly all online media motivated to some extent by views, a publication doesn’t have to be written by teenagers in Macedonia to perpetuate misinformation. The very structure of the web enables what BuzzFeed’s head of data science calls “not-fake-but-not-completely-true information”.

Humour sites, too, can be easily taken at face value, especially if they touch on current events or politics and if they appear free of context on social media. The Onion, Clickhole and the New Yorker’s Borowitz Report are often taken at face value – Clickhole in particular has the spirit of fake news about it, with its tagline: “because all content deserves to go viral”.

It’s that quality of “truthiness” again. “#NotTheOnion” has become a popular catch cry for real stories that seem so ironic or ridiculous as to be fake at first glance – for example, “President-Elect tweets that Saturday Night Live is ‘Unwatchable’.” The most successful fake news, as with humour, is deliberately pitched to seem just ridiculous enough to be true.

Is this column or review or news report I disagree with fake news?

Not if it doesn’t meet the definition above. “Fake news” is not a one-size-fits-all rejoinder for editorial decision-making or reporting you disagree with Stick with “that’s not news”.

Melissa Zimdars, an associate professor of communication and media at Merrimack College in Massachusetts, compiled this list of websites that either purposely publish false information or are otherwise entirely unreliable, broken down by category – and published a helpful list of tips for analysing news sources.

But Facebook’s approach has shortcomings and no list can ever be complete. You can’t go wrong by prioritising outlets known to be legitimate, and reading a lot of them. If it is published on the Guardian – just for example – it may well not be news, but it won’t be fake news. (Sorry, Breitbart.)

If you’re not sure if a site is legitimate, look for any red flags in its domain name, such as “.com.co”, and its About Us section. Google the sources of any quotes or figures given in the story – most fake news don’t have either, a warning sign in itself.

If the first you’ve heard of a particular event is from a website you’ve never heard of, there may be a reason. Be sceptical of stories about Trump, Clinton, the Pope, Kim Kardashian and Justin Bieber, and particularly of stories about any of them pledging allegiance to Isis.

And what can I do to stop its spread?

Share responsibly. Much as it might depress you to think in such terms, you are an influencer within your own social network: put in the legwork above, and only post or share stories you know to be true, from sources you know to be responsible. It’s the “take only photographs, leave only footprints” for the post-truth era.

You can help shape the media you want, too. Withhold “hate-clicking” on stories you know are designed to make you angry. Pay for journalism you value.

And if you have connections on Facebook who think Onion stories are real, break it to them gently. Friends don’t let friends share fake news.